There is USB as one of computer interfaces for connecting a host device and a peripheral device. In the USB, a USB controller housed in the host device (also called a master) and the peripheral device (also called a target, a function, or simply a device) are connected in a one-to-one relationship or through a hub. The host device includes, as hardware, a route hub and one or more USB ports (receptacles) connected to the route hub, and as software, an OS and device drivers for bus management and data communication with the peripheral device.
Data transfer between the host device and the peripheral device is performed in such a host and target relationship that only the host device can start the data transfer and the peripheral device returns, to the host device, data corresponding to a request from the host device. The host device is generally a computer, and peripheral devices include a USB memory, a hard disk drive, a keyboard, a mouse, a printer, a digital camera, an audio device, and the like. A USB 2.0 line is made up of a pair of differential data lines (D±), a power line (VBUS), and a ground line (GND). As for a USB 3.0 line, two pairs of differential data lines (Tx±, Rx±) are added to these lines.
For such host/target type USB communication, On-The-Go and Embedded Host Supplement to the USB Revision 2.0 Specification and On-The-Go and Embedded Host Supplement to the USB Revision 3.0 Specification define USB_OTG (USB On-the-Go) that allows any device other than a personal computer (PC), such as a tablet terminal or a smartphone, to act as both a host device and a peripheral device. An OTG device that complies with the USB_OTG specifications is also called a dual-role device in the sense of including both the functions of the host device and the peripheral device.
When the host device is a portable device equipped with a rechargeable battery, such as a smartphone or a tablet terminal, the battery can be charged through a VBUS line with a USB charger directly connected to a receptacle. When the portable device is an OTG device, it is suitable for downsizing because usually only one micro-AB receptacle has to be equipped therein. However, since only either the USB charger or a peripheral device can be connected, charging and communication cannot be performed at the same time. Therefore, a special device is required to use a peripheral device while charging the portable device at the office or at home.
USB Battery Charging Specification Revision 1.2 defines a standard-ACA (Standard Accessory Charger Adapter) for connecting the USB charger and an accessory to a micro-AB receptacle of the portable device (OTG device) at the same time. A standard-ACA 10 shown in FIG. 6 is equipped with a micro-A plug for connection to a portable device 11 and a standard-A receptacle for connection to an accessory 13. The standard-ACA includes a charger switch 19 and an adapter controller 21. The adapter controller 21 declares either a ground state or a floating state to the portable device 11 through an ID_OTG line.
The portable device 11 detects the electric potential of the ID_OTG line to determine an operating mode. When the portable device 11, the accessory 13, and a USB charger 15 are connected to the standard-ACA at the same time, the USB charger outputs voltage to the VBUS line. The portable device 11 can charge an internal battery with power supplied from the VBUS line, and the accessory 13 can operate with the power supplied from the VBUS line. Aside from the standard-ACA, an OTG cable equipped with a micro-A plug for connection to the OTG device, a micro-B receptacle for connecting the USB charger, and a standard-B receptacle for connecting a peripheral device is commercially available. In the OTG cable, the VBUS line to be connected to the OTG device branches for the USB charger and the peripheral device.
Japanese Patent Application Publication No. 2001-306413 discloses a USB cable device for connecting a host device equipped with two USB ports and a peripheral device. The USB cable device connects a two-port VBUS line, to be connected to the host device, through a diode to supplement power from the other port, where the power from one port is insufficient to be supplied to the peripheral device. Japanese Patent Application Publication No. 2012-502354 discloses power control of an OTG device acting as either a host device or a peripheral device.
The OTG device receives either of a host signal HST and a peripheral signal PER from a circuit mechanism for detecting a USB operating mode to determine the operating mode. When the peripheral signal PER is asserted, the OTG device transitions to a peripheral mode. In the peripheral mode, a USB transceiver of the OTG device receives the supply of power through VBUS from a host device connected to a USB port. At this time, power is supplied from a battery to a non-USB circuit mechanism of the OTG device.
When the host signal HST is asserted, the OTG device transitions to a charging mode for charging the capacitance of the VBUS line, and when voltage on the VBUS line rises, the OTG device transitions to a host mode. In the host mode, the battery supplies power to the non-USB circuit mechanism and further supplies power to a receiver of the peripheral device through the VBUS line. Japanese Patent Application Publication No. 2000-357029 discloses a charging device for a portable telephone set. The portable telephone set includes a communication/power connector capable of being commonly used for communication and power supply, and a charging terminal part to be connected to a dedicated charger. A communication interface device inserted between a PC and the portable telephone set is connected to the communication/power connector through a cable including a power line and a signal line to be connected to the portable telephone set.